Project Summary The project goal is to understand the impact of pre-existing dengue virus immunity (both wildtype- and vaccine-derived) on subsequent Zika virus (ZIKV) infection during pregnancy. Understanding the relationship between dengue immunity on subsequent ZIKV infection is important because it has been hypothesized that the unexpectedly high rate of adverse consequences associated with ZIKV infection during pregnancy may be the result of previous infection with one of the four serotypes of dengue virus (DENV). Primary infection with one of the four DENV serotypes is protective against secondary infection with the same serotype. Secondary infection with a different serotype can lead to enhanced disease due to cross-reactive antibodies facilitating enhanced viral replication and skewed immune responses. DENV and ZIKV are similar enough in the envelope protein (the major target of these enhancing antibodies) that ZIKV could theoretically function as a ?fifth? DENV serotype. This has been explored to some degree but the reports have been controversial with some groups reporting cross-protection, while others have reported the potential for enhanced disease. Critically, this has not been explored in the setting of pregnancy. Accordingly, through this NIH/NIAD R01 we will use our nonhuman primate model of ZIKV infection to evaluate whether the severity of maternal and fetal ZIKV infection during pregnancy are enhanced by previous exposure to DENV. There are two Specific Aims: Specific Aim 1. Define the impact of prior DENV infection on the severity of maternal and fetal ZIKV infection in pregnant macaques. Specific Aim 2. Define the impact of prior DENV vaccination on the severity of maternal and fetal ZIKV infection in pregnant macaques. In these studies, we will contrast maternal viremia, immune responses, and fetal outcomes with those in DENV-naive individuals infected identically with ZIKV. Our strategy to induce DENV-specific immune responses includes exposure to both wildtype DENV and a leading DENV vaccine candidate- Sanofi Pasteur?s Dengvaxia. These studies are critically important because 1) ZIKV is circulating in many locations where DENV is hyperendemic and 2) dengue vaccines currently are licensed for use or licensure is imminent in areas where ZIKV is circulating. Vaccination creates a scenario whereby a naive population will become DENV-immune. Whether immunization with Dengvaxia, a tetravalent vaccine that provides simultaneous exposure to all four DENV serotypes, can lead to more severe ZIKV disease is unknown. The results from these experiments will provide important answers to an ep- idemiologically relevant question: is it safe to vaccinate against dengue where ZIKV also is co-en- demic?